The present invention relates to light curtains, in particular safety light curtains, for monitoring, a protective field. Furthermore, the present invention relates to optical units which are part of such a light curtain.
Generally, light curtains detect the movement or intrusion of objects into guarded zones, and more particularly, provide protection for human operators who are working with machines or other industrial equipment.
Light curtains employing infrared or visible light beams are used to provide operator safety in a variety of industrial applications. Light curtains typically are employed for operator protection around machinery, such as punch presses, brakes, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like. Conventional light curtains typically employ light emitting diodes (LED) mounted at spaced positions along a transmitter bar at one side of the guard zone and phototransistors (PT), photodiodes or photoreceivers mounted along a receiver bar at the opposite side of the zone. The LEDs transmit modulated infrared light beams along separate parallel channels to the PTs at the receiver bar. If one or more beams are blocked from, penetration by an opaque object, such as the operator's arm, a control circuit either shuts the machine down, prevents the machine from cycling, or otherwise safeguards the area.
Usually, safety light curtains comprise two optical units (called bars, sticks, or strips), which are formed as two different constructional units, one of the optical units having the functionality of an emitter and one of a receiver. This dedicated architecture of an emitter and receiver, however, has several drawbacks.
Firstly, the fabrication costs are high, because each type of optical unit has to be fabricated differently. Further, due to the fact that the optical communication is only unidirectional, i.e. from the sender to the receiver, the optical synchronization may be difficult and a transmission of communication information is possible only in one direction.
Secondly, when using the light emitting elements of the sender optical unit for performing an alignment of the light curtain, the accuracy is not satisfactory, as only half of the system can be aligned by using this scheme. It is known therefore, to use a separate alignment light source, for instance a laser, as this is explained in the European patent 0889332. However, the use of such an integrated laser alignment system adds complexity and costs to the manufacturing of the optical units.
It has already been proposed to locate receivers and transmitters on each of the first and second optical units, as this is described in the European patent EP 1870734. Here, the grid has two identical transmitting/receiving strips, to which transmitting and receiving units are fixed. The transmitting/receiving strips are placed opposite to each other with a protective field being formed between the strips. The transmitting/receiving strips are identically formed in control and evaluation units. The control and evaluation units have safety outputs, which are formed together as a switching channel. An identical power supply is provided for all the strips. However, due to the fact that the known transmission/reception bars are constructed identically and also operate identically, synchronization is a particularly difficult issue.
Furthermore, due to the significant architectural differences to established systems with separate receiver and sender bars, the combined receiver/transmitter units according to EP 1870734 cannot be used for retrofitting or updating existing light curtains.
Moreover, the system according to EP 1870734 also has the disadvantage that it can only provide a single channel communication, whereas for many applications a dual channel configuration is required.
Furthermore, it is known to provide an additional communication channel for providing a bidirectional communication. As the synchronization algorithm takes a long time to achieve synchronization with only a unidirectional optic, it is known to use alternatively a connection cable and synchronize the sticks electrically.